A private network is a closed communication network provided for carrying out communications between specified users, including a leased line network, a virtual private network, and the like. A number of leased line networks, such as government office networks and enterprise networks, are in use at present. Economical use of a communication network, provision of various communication services, flexibility of servicing, etc., can be easily realized by constructing such a private network.
In a private network, assurance of security becomes an important point, but if a private network is connected with a public network, services can be provided to a number of subscribers without being limited to specific areas. A variety of systems for connecting a public line and a private line have been proposed.
An additional dialing system is one of the conventional systems for connecting a public network subscriber and a leased line network subscriber. For example, when a public network subscriber originates a call and a private branch exchange (PBX), which becomes an access point to a private network, responds to the call incoming, this subscriber additionally dials an authorization code, which is equivalent to a password, to the PBX. The incoming call is authorized by the authorization code. Then, the leased line number of a called party is further additionally dialed, thereby allowing connection to a leased line network subscriber.
However, in this additional dialing system, the calling party has to dial a number of digits and carry out very complicated originating procedures. Moreover, there is a fear of abuse of the authorization code if its number is known by other people. There is a problem in that complete security cannot be secured.
In another conventional system, a dial number transmitting system uses a data terminal having a restriction on the number of dial digits that can be transmitted automatically, which makes the public network--leased line network--public network connection possible (Refer to Japanese patent unexamined publication No. 64-86642.) In this dial number transmitting system, the network control unit (NCU) connected with a data terminal previously stores the identification number of the data terminal and the public network subscriber's number of the nearest PBX which becomes an access point to a leased line network, the PBX having an additional number dial-in function, which can directly call an extension. On receipt of a called party selecting dial number from the data terminal of a calling party, which is necessary for connection with a leased line network and a public network, the NCU selects the nearest PBX and a data call is established through the leased line network and the public network.
In such a dial number transmitting system, however, the security of the leased line network is not taken into account at all because the leased line is regarded as a mere transmission line. Furthermore, the calling party has to dial a called party selecting dial number, which is necessary for connection with a leased line network and a public network. Therefore, the dialing operation is complicated.
An object of the present invention is to provide a connection system of a public network subscriber and a private network subscriber, which can improve the subscriber's operability, and ensure complete security.